1. Field of the Invention
The present invention relates to a travel drive apparatus for a working vehicle such as a large-size transportation vehicle and dump truck which is suitably used in transporting crushed stones excavated from a mine or the like.
2. Description of the Prior Art
Generally, large-size transportation vehicles, called dump trucks, have a liftable vessel (a loading platform) on a frame of a vehicle body, and carry heavy objects to be transported like crushed stones in a state in which the objects to be transported are loaded in a large quantity on the vessel.
In this regard, for example, a travel drive apparatus which is adapted to drive a drive wheel of a dump truck is provided with a tubular axle housing which is mounted on a vehicle body, a drive source such as an electric motor or a hydraulic motor which is provided in the axle housing and rotationally drives a rotational shaft, a wheel mounting case rotatably mounted around an outer periphery on the fore end side of the axle housing through a bearing to support a drive wheel thereon for traveling, and a multi-stage reduction gear mechanism incorporated into the wheel mounting case and is adapted to reduce the speed of rotation of the rotational shaft so as to transmit this rotation to the wheel mounting case (Japanese Patent Laid-Open No. S62-221918 A and 2006-264394 A).
By a multi-stage reduction gear mechanism of this sort, rotational output of a drive source like a electric motor, for example, is reduced in speed and transmitted to a tubular wheel mounting case (wheel) to generate a large rotational torque in a drive wheel of a dump truck, namely in a front or rear wheel to thereby enhance the transporting performance of the dump truck (vehicle). In addition, a travel drive apparatus for a dump truck is normally adapted to rotationally drive left and right wheels by respective reduction gear mechanisms independently of each other.
In order to keep each gear member of the reduction gear mechanism in a lubricated state, the lubricant oil is pooled in a tubular wheel mounting case on which a drive wheel is to be mounted. Further, this lubricant oil is forcibly circulated in and outside the wheel mounting case (axle housing) by using a lubricant pump. Moreover, the lubricant oil is cooled by being subjected to heat exchange by an oil cooler provided in the course of a circulation line.
In this case, for example, a lubricant pump is located on the outer side of the axle housing. A suction pipe is provided by being connected to a suction side of the lubricant pump, with the one side (suction port side) of the suction pipe being extended into and immersed in the lubricant oil pooled in the wheel mounting case. On the other hand, an output pipe or a supply pipe is provided by being connected to the output side of the lubricant pump, and the other side of the supply pipe is disposed so as to be extended into the wheel mounting case at a position above the suction pipe.
Incidentally, with the above-described conventional art, if the travel speed of the vehicle is increased by rotating the wheels at high speed, the lubricant oil pooled in the wheel mounting case is subjected to the influence of centrifugal force accompanying the rotation of that wheel mounting case. For this reason, the lubricant oil behaves in such a manner as to stick to the entire periphery of the inner wall surface of the wheel mounting case, and hence there are cases where the surface level of the lubricant oil drops to a position lower than the suction port of the suction pipe.
Accordingly, when the travel speed of the working vehicle is increased to cause the suction port of the suction pipe disposed in the wheel mounting case to become located away from the surface level of the lubricant oil, the action of sucking the lubricant oil by the lubricant pump becomes impossible. As a result, the lubricant pump assumes an idling state and is driven dry, so that seals, bearings, and the like of the lubricant pump are likely to become worn and damaged at an early period, causing the pump life to become shortened.
Accordingly, in order to prevent the lubricant pump from assuming the idling state, the present inventors studied the feasibility of immediately stopping the drive of the lubricant pump when the travel speed of the vehicle has become faster than a predetermined determination speed. However, in an initial stage of vehicle operation such as during a warm-up operation, the temperature of the lubricant oil is low, so that the viscous resistance becomes large, and the suction characteristic in the suction of the lubricant oil by the lubricant pump undesirably changes due to its dependence on the temperature of the lubricant oil (viscous resistance).
Namely, when the temperature of the lubricant oil is low at a temperature of, for example, not more than 50° C. in the initial stage of vehicle operation, even if the vehicle is traveling at a lower speed than the aforementioned determination speed during steady operation, the suction characteristic based on the lubricant pump becomes instable. As a result, there is a problem in that cavitation is prone to occur due to the suction of air, possibly shortening the pump life.
Meanwhile, also as for the installation position of an oil temperature sensor for detecting the temperature of the lubricant oil, since the configuration provided is such that a reduction gear mechanism is provided in the wheel mounting case, and the lubricant oil is pooled in the interior, it is, in practice, difficult to provide an oil temperature sensor, for instance, inside the wheel mounting case to directly detect the temperature of the lubricant oil, because structural restrictions are large.
For this reason, the present inventors studied the feasibility of providing the oil temperature sensor on the output side of the lubricant pump by being located outside the wheel mounting case. In this case, however, when the lubricant pump is stopped, it becomes impossible to send the lubricant oil pooled in the wheel mounting case to the position of the oil temperature sensor, with the result that it is impossible to directly detect the temperature of the lubricant oil by using the oil temperature sensor.
Therefore, at the time of starting the operation of the working vehicle at a work site, unless the lubricant pump is started once, the temperature of the lubricant oil cannot be detected by using the oil temperature sensor. For this reason, when the travel of the vehicle has carried on without detecting the temperature of the lubricant oil, there is a problem in that even if the temperature of the lubricant oil actually rises, since the supply control of the lubricant oil is not effected, it is impossible to allow the cooling action based on the lubricant oil to be demonstrated.